The present invention relates generally to capacitive power supplies, and, more particularly to a capacitive power supply having a charge equalization circuit which ensures that the electrical charge collected and stored by the capacitive power supply does not exceed a pre-determined leve.
In order to permit operation of an electrical device at a location not proximate to a permanent power source, a portable power supply is positioned proximate to the electrical device. Once the portable power supply and the electrical device are suitably connected theretogether, the portable power supply may be utilized to power the electrical device thereby. Conventionally, a portable power supply is formed of an electrochemical material, and energy is stored by the electrochemical material in the form of chemical energy. Power required to operate the electrical device is formed by converting the stored, chemical energy of the electrochemical material into electrical energy.
Such a conventional, electrochemical power supply is commonly referred to as a battery, and one or more commercially-available batteries may be utilized to generate a direct current voltage to power many varied types, constructions, and designs of electrical devices. Such conventional, electrochemical battery power supplies are widely available, and, therefore, are conveniently utilized to form the portable power supply to power an electrical device thereby.
A conventional, electrochemical battery is, however, of a finite energy storage capacity. Therefore, a single battery (or several batteries connected theretogether) may be utilized to power an electrical device for only a limited period of time. As the stored, chemical energy of the electrochemical material is converted into electrical energy to power the electrical device, the battery becomes discharged as the stored chemical energy contained by the electrochemical material becomes dissipated.
Once the remaining, stored energy of the battery is depleted below a certain level, replacement of the battery is necessitated to permit continued operation of the electrical device. The frequency with which the battery must be replaced, is, of course, dependent upon the battery capacity (i.e., the amount of energy stored in the battery), the energy required to operate the electrical device, and the frequency with which the electrical device is operated. Other types of portable power supplies are similarly of a finite energy storage capacity, and, therefore, similarly become depleted of stored energy after a period of use thereof.
A battery-type construction comprised of a nickel-cadmium (Ni-Cd) material is often utilized to power an electrical device. The use of the nickel-cadmium material to form the electrochemical material is advantageous for the reason that, once a battery formed therefrom is depleted of stored energy, an electrical, charging current may be applied to the battery to recharge the battery thereby. Battery-type constructions comprised of other materials may similarly be recharged once depleted of stored energy by the application of a charging current thereto.
Battery-type constructions comprised of a nickel-cadmium material (as well as other such similar materials) are not of unlimited battery-lifes. That is to say, a rechargeable battery comprised of such a material cannot be recharged and reused an unlimited number of times. Over time, as the nickel-cadmium battery is discharged, and subsequently recharged, the efficiency of energy conversion of electrical energy (supplied by the charging current to recharge the battery) into stored chemical energy of the electrochemical material is reduced. Over time, such reduction in efficiency of energy conversion makes impractical continued reuse of the same battery. Once the battery may no longer be efficiently recharged, the battery must be discarded and replaced with a battery capable of being efficiently recharged to permit continued powering of the electrical device.
Additionally, when recharging a rechargeable battery, the rate at which the charging current is applied to the battery must be controlled. If the charging current applied to the battery to recharge the battery thereby is beyond a certain level, the battery may be damaged by the charging current. The maximum, allowable level of charging current which may be applied to various battery-type constructions to recharge the battery thereby varies. For instance, a rechargeable battery comprised of the aforementioned nickel-cadmium material may be recharged with a charging current of 600 millamperes. A battery-type construction comprised of a lithium material (another material of which a rechargeable battery may be comprised), conversely, cannot be charged with a charging current in excess of 100 milliamperes. Battery charging apparatus utilized to provide the rechargeable battery with the charging current to charge to the battery thereby cannot be utilized to recharge rechargeable batteries of the various battery-type constructions without appropriate alteration of the level of the charging current.
Further, because the level of the charging current applied to the rechargeable battery cannot exceed a certain, maximum level, the charging rate of the battery cannot be increased beyond the allowable level, and the time required to recharge a battery, once discharged to a certain level, cannot be reduced below a certain minimum time period. If only one rechargeable battery is available to power the electrical device, the electrical device cannot be operated once the battery has been discharged beneath a minimum level, and the time period required to recharge the battery to a level above the minimum level to permit continued operation of the load element, cannot be reduced less than the minimum time period.
Still further, when charging a rechargeable battery, such as a nickel-cadmium battery, with a maximum, allowable charging current, once the battery becomes fully charged, application of the charging current at the high charging level to the battery must be terminated. Continued charging of the battery at the high charging rate can cause gassing, electrolytic venting of the battery, permanent loss of battery capacity, and physical damage to the battery.
Other constructions of portable power sources are known, but heretofore have been practical for only low power applications. In particular, capacitive power sources have been utilized as back-up power supplies for integrated circuit memories to power the integrated circuits for short periods of time, such as may occur during temporary power interruptions.
Capacitors having capacitances of values great enough to generate current levels to power most electrical devices (e.g., a consumer electronic device such as a portable radiotelephone) for extended periods of time have previously been impractical for the reason that the capacitors forming such capacitive power supplies would be of prohibitively large dimensions. Other of such capacitive power sources are of very high effective resistances, and are similarly impractical for use to generate large current levels. However, co-pending application Ser. No. 596,253, filed on Oct. 12, 1990, and entitled "Capacitive Power Supply" by Metroka et al. discloses a capacitive power supply capable of generating significant current levels for an extended period of time.
The capacitive power supply disclosed therein is capable of generating a continuous current level in excess of 600 milliamperes for a time period in excess of one and one half hours. Once the charge stored by the capacitive power supply has been depleted through discharge of the stored energy of the supply, the capacitive power supply may be quickly recharged by application of a charging current thereto.
When several of the capacitive elements forming the capacitive power supply are connected in series, any variance in the capacitive values of the capacitive elements results in unequal amounts of charge being stored by various ones of the capacitive elements when a charging current is applied thereto. Termination of application of the charging current once a first of the capacitive elements becomes fully charged prevents all of the capacitive elements from being fully charged. Conversely, continued application of the charging current to the circuit after the first of the capacitive elements has been fully charged is inefficient, and could result in arcing of charge across plates of the capacitive elements, or even permanent damage to the capacitive elements.
What is needed, therefore, is a capacitive power supply which may be efficiently charged to a pre-determined level.